Travis, nearly all univalve mollusks (“snails”) are right handed
(called “dextral”). The whorls are clockwise, the aperture is on the
right. There are very few exceptions. One is the Perverse Whelk
(Busycon perversum), uncommon and native to S.E. and eastern Mexico.
Another is the Lightning Whelk (Busycon contrarium), more common and
native to the eastern US. (Left handed shells are called
"sinistral".) If you were able to find a shell which was sinistral,
you would still be unable to find a matching shell of the same species
that coiled in the opposite direction.
Many bivalve mollusks (clams, scallops, etc.) also have very a faint
shallow spiral to their textural patterns, that originates from the
hinge area. The opposing halves will have patterns which “spiral”
(“curve” would be a more accurate word) in opposite directions. Afraid
this is the closest you’re going to get to “matching and opposite”.
What Ive says about going to the opposite coast to get “opposite
opening shells” may be true for certain species of scallops (although
there are Pacific scallops which have wings on both the right and the
left). However, it isn’t true that if you go to the other coast the
whorls of a univalve will switch to being left handed. Nope. Taint so.
I am curious about the shells of the Southern hemisphere, though. Do
they coil in the opposite direction like water going down a drain?
Beachcomber DIW (Dyed In the Wool)